This is normal. The MX Record Lookup failed, was rejected, or couldn't be made to begin with. Just ignore it. Some hosts won't allow the lookups outbound and many mail servers will simply ignore the requests when received. It's a non-stopping warning so you should be able to save users perfectly fine regardless of what it says.
even though the CB Config | Registration | Ajax Email Checker is now set to No. Is there a separate parameter to control this in the backend?
You can't turn ajax checkers off in backend, that parameter only affects frontend.
krileon
